In a heavy duty pneumatic tire for use in heavy duty cars such as truck and bus, a rib type tread pattern is usually due to its superior wear resistance. However, uneven wear, such that only a particular rib wears more rapidly than other ribs, occurs. As a result of the fact that a difference in the tire outer diameter between a tread crown portion and shoulder portions does not decrease when contacting with the ground, the tread rubber in the shoulder portion is dragged on the road surface during running, and so-called diameter difference wear is caused. This diameter difference wear expands in the tire axial direction, especially axially inwards, and a rib in the shoulder portion wears greatly in comparison with that in the tread crown portion. This type of wear is called shoulder wear.
To prevent such uneven wear, a technique shown in FIG. 7 has been proposed. According to this technique, a pneumatic tire having a rib type tread pattern is provided in its shoulder rib (a) with a narrow width groove (c) having a distance from a tread edge (b) which is specifically defined. The shoulder rib (a) is thus divided into an inner rib part (d) and an outer rib part (e). This is designed to sacrifice the outer rib part (e) with less rigidity for the above-explained diameter difference wear to thereby prevent the progress of uneven tread wear towards the inner rib part (d).
Further, another technique shown in FIG. 8 has been proposed, wherein a pneumatic tire having a pattern based on a rib type tread pattern is provided with a rib (f) which is located inside and next to the shoulder rib (a) and the rib (f) is provided in its axially outer edge portion with a narrow-width circumferentially-continuous groove (g). The rib (f) is thus divided into a main rib part (h) and a narrow rib part (i). This is designed to prevent uneven wear of the rib (f) which is the next to the shoulder rib by concentrating the wear on the narrow rib part (i).
Thus, the point of the above-mentioned two propositions is to concentrate wear on a narrow rib part which is formed by disposing a narrow width groove in a rib, wherein the narrow rib part is continuous in the tire circumferential direction and has substantially the same height as the tread surface (j) along the whole length thereof.
However, the narrow rib on which wear is to be concentrated, collapses greatly and is axially deformed and twisted in the ground contacting patch when the rib receives a sideforce from the road surface during cornering which is generally a strong force due to its shape. Those phenomena cause cracks starting from the bottom of the narrow groove and root part of the narrow rib, and tearing off such that the narrow rib is cut into small blocks and some of them drop out.
Thus, there is a problem that wear can not be concentrated on the narrow rib and the rib can not display the wear preventing function, contrary the expectation.